Organic semiconductors based on molecular and polymeric materials have become a major part of the electronics industry in the last 25 years as a complement to the shortcomings of inorganic semiconductors. Most notably, organic semiconductors offer, with respect to current inorganic-based technology, greater ease in substrate compatibility, device processability, flexibility, large area coverage, and reduced cost; as well as facile tuning of the frontier molecular orbital energies by molecular design. A key device used in the electronic industry is the field-effect transistor (FET) based on inorganic electrodes, insulators, and semiconductors. FETs based on organic semiconductors (OFET) may find niche applications in low-performance memory elements as well as integrated optoelectronic devices, such as pixel drive and switching elements in active-matrix organic light-emitting diode (LED) displays.
The thin-film transistor (TFT), in which a thin film of the organic semiconductor is deposited on top of a dielectric with an underlying gate (G) electrode, is the simplest and most common semiconductor device configuration. Charge-injecting drain-source (D-S) electrodes providing the contacts are defined either on top of the organic film (top-configuration) or on the surface of the FET substrate prior to the deposition of the semiconductor (bottom-configuration). The current between S and D electrodes is low when no voltage is applied between G and D electrodes, and the device is in the so called ‘off’ state. When a voltage is applied to the gate, charges can be induced into the semiconductor at the interface with the dielectric layer. As a result, the DS current increases due to the increased number of charge carriers, and this is called the ‘on’ state of a transistor. The key parameters in characterizing a FET are the field-effect mobility (μ) which quantifies the average charge carrier drift velocity per unit electric field and the on/off ratio (Ion:Ioff) defined as the D-S current ratio between the ‘on’ and ‘off’ states. For a high performance OFET, the field-effect mobility and on/off ratio should both be as high as possible.
Most of the OFETs operate in p-type accumulation mode, meaning that the semiconductor acts as a hole-transporting material. However, for the full development of the field of organic semiconductors, high-performing electron-transporting (n-type) materials are needed as well. For most practical applications, the mobility of the field-induced charges should, optimally, be >0.1-1 cm2/Vs. To achieve high performance, the organic semiconductors should also meet or approach certain criteria relating to both the injection and current-carrying phenomena, in particular: (i) HOMO/LUMO energies of individual molecules (perturbed by their placement in a crystalline solid) at levels where holes/electrons may be added at accessible applied voltages, (ii) a crystal structure of the material with sufficient overlap of the frontier orbitals (m stacking and edge-to-face contacts) for charge migration among neighboring molecules, (iii) a compound with minimal impurities as charge carrier traps, (iv) molecules (in particular the conjugated core axes) preferentially oriented with their long axes close to the FET substrate normal, as efficient charge transport occurs along the direction of intermolecular π-π stacking, and (v) uniform coverage of the crystalline semiconductor domains between source and drain contacts, preferably with a film having preferably with a film exhibiting a single crystal-like morphology.
Among n-type organic semiconductors used in OFETs, the class of arene core diimides is one of the most investigated. The first report on a diimide-based FET was on a series of naphthalene tetracarboxylic diimides, followed by reports of perylene tetracarboxylic diimides. Over the years, chemical modification and tailoring of the imide position has resulted in the production and testing of a library of diimide-based materials. However, such compounds have been found generally to be unstable in air and have solubility characteristics less than satisfactory for efficient device fabrication.